Hybrid DNA prepared binding composition

ABSTRACT

Proteinaceous binding compositions are prepared employing hybrid DNA  techogy, where the variable region polypeptides of immunoglobulins are substantially reproduced to provide relatively small protein molecules having binding specificity and lacking the undesirable aspects of the heavy regions of immunoglobulins. The compositions find a wide range of use, particularly for physiological purposes for diagnosis and therapy. The binding compositions may be modified by labeling with radioisotopes, fluorescers, and toxins for specific applications in diagnosis or therapy.

This is a continuation of application Ser. No. 358,414, filed Mar. 15,1982, now abandoned.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The mammalian immunological system is unique in its broad ability toproduce protein compounds having extremely high specificity for aparticular molecular structure. That is, the proteins or immunoglobulinswhich are produced have a conformation which is specifically able tocomplement a particular structure, so that binding occurs with highaffinity. In this manner, the mammalian immune system is able to respondto invasions of foreign molecules, particularly proteins in surfacemembranes of microorganisms, and toxins, resulting in detoxification ordestruction of the invader, without adverse effects on the host.

The primary immunoglobulin involved in the defensive mechanism isgamma-globulin (IgG). This immunoglobulin, which is a glycoprotein ofabout 150,000 daltons, has four chains, two heavy chains and two lightchains. Each of the chains has a variable region and a constant region.The variable regions are concerned with the binding specificity of theimmunoglobulin, while the constant regions have a number of otherfunctions which do not directly relate to the antibody affinity.

In many situations it would be desirable to have molecules which aresubstantially smaller than the immunoglobulins, while still providingthe specificity and affinity which the immunoglobulins afford. Smallermolecules can provide for shorter residence times in a mammalian host.In addition, where the immunoglobulin has to be bound to anothermolecule, it will be frequently desirable to minimize the size of thefinal product. Also there are many economies in being able to produce asmaller molecule which fulfills the function of a larger molecule.

There are situations where it will be desirable to be able to have alarge number of molecules compactly held together. By having smallermolecules, a greater number can be brought together into a smallerspace. Furthermore, where the binding molecule can be prepared by hybridDNA technology, one has the opportunity to bind the binding portion ofthe molecule to a wide variety of other polypeptides, so that one canhave the binding molecule covalently bonded at one or both ends to apolypeptide chain.

Where immunoglobulins are used in in vivo diagnosis or therapy, antiserafrom an allogenic host or from a monoclonal antibody may be immunogenic.Furthermore, when conjugates of other molecules to the antibody areemployed, the resulting conjugate may become immunogenic and elicit hostantibodies against the constant region of the immunoglobulin or againstany other part of the molecule.

It is therefore important that methods be developed which permit thepreparation of homogeneous compositions having high specificity for aparticular ligand, while avoiding the shortcomings of completeimmunoglobulins, and providing the many advantages of lower molecularweight.

2. Description of the Prior Art

Discussions concerning variable regions of heavy and light chains ofimmunoglobulins may be found in Sharon and Givol, Biochem. (1976)15:1591-1594; Rosenblatt and Haber, Biochem. (1978) 17:3877-3882; andEarly and Hood, Genetic Engineering (1981) 3:157-188. Synthesis of partof a mouse immunoglobulin light chain in a bacterial clone is describedby Amster et al., Nucleic Acids Res. (1980) 8:2055-2065. See also thereferences cited throughout the specification concerning particularmethodologies and compositions.

SUMMARY OF THE INVENTION

Novel protein complexes are provided by producing homogeneouscompositions defining the variable regions of the light and heavy chainsof an immunoglobulin, which individually or together form a specificbinding complex to a predetermined haptenic or determinant site.Employing hybrid DNA technology, cDNA is tailored to remove nucleotidesextraneous to the variable regions of the light and heavy chains. Theresulting tailored ds cDNA is inserted into an appropriate expressionvector which is then introduced into a host for transcription andtranslation. The resulting truncated light and heavy chains define atleast a major portion of the variable regions and are combined to form acomplex capable of specifically binding to a predetermined haptenic sitewith high affinity.

DESCRIPTION OF THE SPECIFIC EMBODIMENTS

The subject invention concerns a hybrid DNA strategy for the preparationof specific binding polypeptides, normally comprised of two differentpolypeptide chains, which together assume a conformation having highbinding affinity to a predetermined ligand or haptenic site thereof. Thepolypeptide chains form binding sites which specifically bind to apredetermined ligand to form a complex having strong binding between theligand and the binding site. The binding constant or avidity willgenerally be greater than 10⁵, more usually greater than 10⁶, andpreferably greater than 10⁸. The haptenic binding site or determinantbinding site of the polypeptide chain may be associated with a hapten orantigen.

One or both of the different polypeptide chains derived from thevariable region of the light and heavy chains of an immunoglobulin maybe used to provide specific binding to a ligand. For the most part eachof the polypeptide chains of the light and heavy variable regions wouldbe employed together for binding to the ligand. In describing thisinvention, it will be understood that while the two different chains areindicated as forming a complex, either of the chains could be usedindividually, where feasible due to sufficient binding affinity of theparticular chain to the reciprocal ligand.

The two polypeptide chains which, individually or together, provide thecompositions of this invention will form a receptor site, analogous tothe binding site of an immunoglobulin. The composition will be referredto as an rFv with the individual chains referred to as L-rFv or H-rFv.The L- and H- designations will normally mean light and heavyrespectively, but in some instances the two chains may be the same andderived from either the light or heavy chain sequences. The polypeptidechains of the rFv will generally have fewer than 125 amino acids, moreusually fewer than about 120 amino acids, while normally having greaterthan 60 amino acids, usually greater than about 95 amino acids, moreusually greater than about 100 amino acids. Desirably, the H-rFv will befrom about 110 to 125 amino acids while the L-rFv will be from about 95to 115 amino acids.

The amino acid compositions will vary widely, depending upon theparticular idiotype involved. Usually there will be at least twocysteines separated by from about 60 to 75 amino acids and joined by adisulfide bond to form cystine. The two chains will normally besubstantial copies of idiotypes of the variable regions of the light andheavy chains of immunoglobulins, but in some situations it may besufficient to have combinations of either the light or the heavyvariable region chains.

In many instances, it will be desirable to have one or both of the rFvchains labeled or bound to a support. Various labels may be employed,such as radioisotopes, fluorescers, or toxins. In other situations, oneor both of the chains may be bound to an inert physiologicallyacceptable support, such as synthetic organic polymers, polysaccharides,naturally occurring proteins, or other nonimmunogenic substances.

In some situations, it may be desirable to provide for covalentcrosslinking of the two chains, which could involve providing forcysteine residues at the carboxyl termini. The chains will normally beprepared free of the constant regions, including or being free of all ora portion of the J region. The D region will normally be included in thetranscript of the H-rFv.

For the most part only a relatively small percent of the total aminoacids will vary from idiotype to idiotype in the rFv. Therefore, therewill be areas providing a relatively constant framework and areas thatwill vary, namely, the hypervariable regions.

The C-terminus region of the rFv will have a greater variety ofsequences than the N-terminus and, based on the present strategy, can befurther modified to permit variation from the naturally occurring heavyand light chains. A synthetic oligonucleotide can be employed to varyone or more amino acids in a hypervariable region.

The preparation of the rFv employing hybrid DNA technology will now bedescribed in greater detail.

The preparation of the rFv will be divided into three parts: (1)isolation of appropriate DNA sequences; (2) introduction of the DNAsequences coding for the members of the rFv into an appropriateexpression vector; and (3) expression and isolation of the mimeticvariable regions of the light (L-rFv) and heavy (H-rFv) chains toprovide the rFv.

I. Isolation of appropriate DNA Sequences.

In preparing the DNA sequences, a source of the genes encoding thevariable region will be required. The variable regions may be derivedfrom IgA, IgD, IgE, IgG or IgM, most commonly, from IgM and IgG. Thiscan be achieved by immunizing an appropriate vertebrate, normally adomestic animal, and most conveniently a mouse. The immunization may becarried out conventionally with one or more repeated injections of theimmunogen into the host mammal, normally at two to three week intervals.Usually three days after the last challenge, the spleen is removed anddissociated into single cells to be used for cell fusion to providehybridomas.

The immunogen will be the antigen of interest, or where a hapten, anantigenic conjugate of the hapten to an antigen.

In order to prepare the hybridomas, the spleen cells are fused underconventional conditions employing a fusing agent, e.g. PEG6000, to avariety of inter- or intraspecies myeloma cells, particularly mousecells such as SP-2/0, NS-1, etc. and then suspended in HAT selectivemedia. The surviving cells are then grown in microtiter wells andimmunologically assayed for production of antibodies to the determinantsite(s) of interest.

Assays for antibodies are well known in the art and may employ a varietyof labeled antigens or haptens, where the labels are convenientlyradioisotopes, fluorescers, enzymes, or the like. Other techniques mayalso be employed, such as sandwich techniques involving two antibodies,one bound to a support and the other being labeled. The cells frommicrotiter wells scored as positive are cloned either by limitingdilution or cloning in soft agar. The resulting cloned cell lines arethen propagated in an appropriate nutrient medium and, if necessary, maybe stored frozen in liquid nitrogen.

After selection of a particular cell line providing a monoclonalantibody of interest, the cells are expanded. Conveniently, the cellsmay be grown to a density of about 1×10⁶ cells/ml in a 1 L culture. Thecells are then harvested by centrifugation and lysed.

In order to obtain the desired DNA sequence, one can look to either thegene expressing the variable region or the messenger RNA, whichexpresses the variable region. The difficulty with employing genomic DNAis in juxtaposing the sequences coding for the variable region, wherethe sequences are separated by introns. One must isolate the DNAfragment(s) containing the proper exons, excise the introns and thensplice the exons in the proper order and orientation. For the most part,this will be difficult, so that the alternative technique employing themessenger RNA will be the method of choice.

Where the messenger RNA is to be employed, the cells will be lysed underRNase inhibiting conditions. The messenger RNA has the advantage thatthe mature messenger is free of introns, so that the sequence iscontinuous for the entire variable region. Difficulties with messengerRNA have been encountered, due to incomplete reverse transcription butthese difficulties can be minimized. The first step is to isolate themessenger RNA. Conveniently, messenger RNA can be separated from otherRNA because of its polyadenylation, employing an oligo-(dT) cellulosecolumn. The mixture of messenger RNAs will be obtained free of otherRNA. The presence of messenger RNAs coding for the heavy and light chainpolypeptides of the immunoglobulins may then be assayed by hybridizationwith DNA single strands of the appropriate genes. Conveniently, thesequences coding for the constant portion of the light and heavy chainsmay be used as probes, which sequences may be obtained from availablesources (see, for example, Early and Hood, Genetic Engineering, Setlowand Hollaender eds. Vol. 3, Plenum Publishing Corp., New York (1981),pages 157-188.)

Whether the messenger RNA codes for the correct immunoglobulin may bedetermined by in vitro translation employing a rabbit reticulocytecell-free extract (Pelham and Jackson, Eurp. J. Biochem. (1976)66:247-256). The resulting translation product may then be isolated byemploying antibodies specific for one or more of the regions of thechain of interest, for example, using rabbit anti(mouse IgG) where thechains are derived from mouse immunoglobulin.

The immunoprecipitate may be further analyzed by polyacrylamide gelelectrophoresis, and the presence of complexes determined by usingradiotagged receptors for antigen-antibody complexes, such as S. aureusprotein A, Rf factor, or the like. In addition, RNA blot hybridizationcan be employed to further insure that the correct messenger RNA ispresent.

The crude mixture of mRNA sequences containing the desired mRNAsequences will be treated as follows. In order to enhance theprobability that full length cDNA is obtained, the method of Okayama andBerg, Mol. Cell. Biol. (1982) may be employed. Alternatively, themethods described by Efstradiadis and Villa-Komaroff (1979) in GeneticEngineering: Principles and Methods 1, Setlow and Hollaender, eds., NewYork, Plenum Press, pages 15-36, or Steinmetz et al. (1981) Cell24:125-134, may be employed. The first strand of cDNA is preparedemploying a virus reverse transcriptase in the presence of primer. Asecond strand may then be prepared employing reverse transcriptase, theKlenow fragment of DNA polymerase I or T4 polymerase. If necessary, theresulting ds cDNA may then be treated with a single-strand-specificnuclease, such as S1 nuclease for removal of single stranded portions toresult in ds cDNA, which may then be cloned.

II. Preparation of Genes Coding For L-rFv and H-rFv and Introductioninto an Expression Vector For Amplification.

A wide variety of vectors may be employed for amplification orexpression of the ds cDNA to produce the light and heavy chains of theimmunoglobulin. A vector having an appropriate restriction site isdigested with the appropriate endonuclease. The ds cDNA obtained fromthe reverse transcription of the mRNA may be modified by ligatinglinkers, treatment with terminal transferase or other techniques toprovide staggered (complementary) or blunt ended termini. The vectorsmay have one, two or more markers for selection of transformants.Desirably, the vector will have a unique restriction site in one ofmultiple markers, so that the transformants may be selected by theexpression of one marker and the absence of expression of the othermarker. Various markers may be employed, such as biocide resistance,complementation of an auxotroph, viral immunity, or the like.

After transforming an appropriate host with the ds cDNA prepared fromthe mRNA, e.g. E. coli, B. subtilis, S. cerevisiae, etc., in accordancewith conventional ways, the transformants are plated and selected inaccordance with the particular markers. The resulting colonies arescreened, by restriction electrophoretic pattern, hybridization to alabeled probe or by any other conventional means. See, for example,Hanahan and Meselson (1980), Gene 10:63-67. One procedure employs colonyhybridization, where the transformants are grown on a solid medium toproduce colonies. Cells from the colonies are transferred to anitrocellulose replica filter, the transferred cells incubated forfurther growth, lysed, dried and baked. The replica filter is thenhybridized with appropriate radioisotope labeled probes. Conveniently,there are readily available probes for the determinant sites present inthe constant regions of a variety of mammalian immunoglobulins. Thecolonies may be probed based on the nature of the particularimmunoglobulin, as well as the different determinant sites, which may bepresent with the particular immunoglobulin.

The host colonies, usually bacterial, which have DNA which hybridizes toeither the light or heavy chain probes are picked and then grown inculture under selective pressure. In order to maintain selectivepressure, it is desirable that the vector which is employed havebiocidal, particularly antibiotic, resistance. After sufficient time forexpansion of the host, the host cells are harvested, conveniently bycentrifugation. The hybrid plasmid DNA may then be isolated by knownprocedures. (Gunsalus et al., J. Bacteriol. (1979) 140:106-133).

The isolated plasmid DNA is then characterized by restriction enzymedigestion and DNA sequence analysis. These analyses insure that theisolated cDNA clones completely encode the variable region and,optionally, the leader sequences for the light or heavy chain of thedesired immunoglobulin. Furthermore, by having a restriction map of thevariable regions and leader sequences, as well as the flankingsequences, one can determine the appropriate restriction sites forexcising a DNA fragment which will allow for appropriate modification ofthe DNA sequence for insertion into a vector and expression of thepolypeptide of interest. Where no unique restriction site is availableat an appropriate position in the flanking regions, partial digestionmay be employed, with selection of fragments having the variable regionand, optionally, the leader sequence intact. Where the 5' and 3'flanking regions are too extended, these can be chewed back using Bal 31to varying degrees by varying the period of digestion.

Furthermore, by knowing the DNA sequence of the coding strand in theregion of the C-terminus of the heavy and light chain variable regions,a stop codon may be introduced at the C-terminus by the followingprocedure of in vitro mutagenesis. The cDNA is restricted with theappropriate enzyme(s) to provide a variable region coding segment withadditional 5' and 3' flanking sequences. This segment is purified, forexample, by gel electrophoresis, gradient density centrifugation, etc.After isolating the desired segment, the two strands of the segment aredissociated, conveniently by boiling. Alternatively, the undesiredstrand of the intact cDNA-plasmid clone may be nicked and digested.

A synthetic, single-stranded DNA oligomer is prepared, conveniently bysynthesis, which will have at least about 12 nucleotides, more usuallyabout 15 nucleotides, and will generally have fewer than about 50nucleotides, usually fewer than 30 nucleotides, since a more extendedoligomer is not required.

Where heteroduplexing is involved, the non-complementary nucleotideswill usually be flanked by at least about three, more usually at leastabout six nucleotides complementary to the hybridized strand. Theheteroduplexing oligonucleotide will be complementary to the sequence ator about a significant juncture i.e. between the leader sequence and thevariable region or the variable region and the constant region. Thesynthetic DNA oligomer will be complementary to the coding ("sense")strand of the variable-region sequence, but altered to encode atermination codon at the C-terminus of the variable region. That is, theoligomer will be complementary to the coding strand except at or aboutthe amino acid which is involved at the juncture of the variable regionand the D-, J- or C-regions of the light and heavy chains, particularlyat or intermediate the D- or J-regions or intermediate the J-region, orat the J- region and C-region juncture. It is intended that there willbe some variation in the polypeptides which are prepared, so far asextending beyond the variable domains or not including all of the aminoacids at the C-terminus of the variable region.

An excess amount of the oligomer is combined with the denatured strandsof the restriction fragment under sufficiently stringent hybridizationconditions. Thus, the oligomer specifically heteroduplexes to thecomplementary portions of the coding strand, while providing one or morestop and/or nonsense codons to insure the termination of expression atthe desired amino acid at the C-terminus.

After sufficient time for hybridization at the desired level ofstringency, sufficient amounts of the four deoxynucleotides are added inconjunction with the Klenow fragment of DNA polymerase I. A strandcomplementary to the coding sequence of the variable-region and any5'-flanking sequence is synthesized by enzymatic elongation of theprimer resulting in a sequence complementary to the strand to which theoligonucleotide is bound. The single-stranded DNA sequence on the codingstrand located 3' to the region hybridized to the syntheticoligonucleotide is degraded by the 3'-5' exonuclease activity of the DNApolymerase. In this manner, ds cDNA is obtained which specifically codesfor the variable-region and upstream flanking regions associated withthe light and heavy chains. Each of the heavy and light chains isencoded to terminate expression at a predetermined codon in the V, D orJ region.

The resulting heteroduplexed blunt-ended ds cDNA fragments are thenemployed for preparation of homoduplexed ds cDNA coding for the lightand heavy variable regions with the stop codons at the desired sites.Conveniently, the blunt ended fragments are modified as describedpreviously, e.g. joined to linkers which code for restriction siteswhich are absent in the variable region sequences, or may be tailed e.g.polyG or polyC, or used directly for insertion. With restriction sitelinkers, after insertion of the fragment into an appropriate vectorhaving complementary termini, the fragment can be recovered byrestriction at the linker sites. The linkers are joined to the codingsequences with an appropriate ligase, e.g. T4 ligase, the resultingfragment restricted to provide cohesive ends, and the product annealedto the complementary ends of a vector.

At this stage, the vector which is employed provides for amplificationand convenient isolation of transformants having the variable regioncoding sequence insert. Numerous vectors for amplification in bacteriaor other hosts exist such as pBR322, pSC101, pRK290, 2μ-plasmid, etc.The hybrid plasmid containing the mismatched sequences will replicate inthe host to generate two different plasmid molecules, one with theoriginal sequence and one with the "tailored" or "site mutated" sequencederived from the synthetic oligonucleotide. Therefore, each transformantcolony is grown in small (approximately 2 ml) culture for plasmidisolation.

The transformants are grown, the plasmid DNA isolated in accordance withknown procedures, and used for a second cycle of transformation toprovide individual clones replicating the tailored sequence. The clonesmay be screened by filter blot hybridization, probing with a labeledsynthetic oligonucleotide which will include the syntheticoligonucleotide employed in tailoring the variable region sequence, orother convenient technique. Thus, plasmids are obtained having ds cDNAflanked by appropriate restriction sites and having a stop codon at apredetermined site.

Having now defined the 3'-terminus of the coding strand or,alternatively, the C-terminus amino acid, the 5'-region or N-terminus ofthe polypeptide is now defined. Of course, the particular order in whichthe two termini are modified is primarily one of convenience, and caneven be done simultaneously, where primer repair is used at the 5'-endof the coding strand in conjunction with site mutation at the 3'-end.

Different strategies may be evolved, depending upon the nature of thehost in which expression is to be obtained, and whether such hostrecognizes the leader sequence as a secretory signal for secretion ofthe polypeptide with concomitant removal of the leader sequencepolypeptide. Where this opportunity is not available, the strategy willinvolve removal of the leader sequence to provide a start codon at the5'-terminus of the sequence of the coding strand coding for the variableregion, which sequence can be inserted into an expression vector, so asto be under the control of a predetermined promoter and ribosomal startsite.

Based on the sequence of the leader region or the sequence coding forthe N-terminus of the variable region, different oligonucleotides forhomo- or heteroduplexing can be prepared.

Where the leader sequence is retained, primer repair is employed toremove the 5'-flanking sequence of the coding strand. When the primerrepair of the N-terminus is performed simultaneously with the C-terminusmutagenesis, after treatment with the DNA polymerase, the resultingpartial double stranded DNA will be treated with a 5'-3'-single strandexonuclease to remove the 5'-flanking region as well as a ligase toprovide for covalent linking of the replicated strand to the N-terminusoligonucleotide.

Where the leader sequence is to be removed, in vitro mutagenesis isemployed to introduce an f-met codon at the N-terminus of the DNAsequence coding for the variable region.

Alternative strategies may be employed for recovering the desired dscDNA and performing the in vitro mutagenesis. If useful restrictionsites are distant from the coding regions, the plasmid may be digestedwith the appropriate restriction endonuclease, followed by digestionwith a double-strand exonuclease e.g. Bal 31. The resulting ds cDNA maybe cloned and the proper sequence selected and modified, as appropriate,as described above. If the noncoding flanking region at the 5'-terminusof the coding strand is too long, it may be digested with anendonuclease, where a convenient restriction site is available or bydigestion with an exonuclease e.g. Bal 31.

By repeating the above described procedure for modifying the3'-terminus, except that the oligonucleotide is now complementary to thenon-coding (nonsense) strand, and includes an initiation codon at the5'-end (primer repair) or within the oligonucleotide (in vitromutagenesis), the 5'-terminus of DNA sequence encoding the variableregions may be tailored. Normally, the oligonucleotide homoduplexes forprimer repair and heteroduplexes for in vitro mutagenesis. In this way,"tailored" ds-cDNA is obtained which has start and stop codons properlypositioned to define the variable regions of both the light and heavychains of immunoglobulins. The resulting blunt ended ds cDNA may bemodified, e.g. by addition of linkers, to provide complementary terminifor insertion into an expression vector in proper spacing to theregulatory signals which are ligated to the ds cDNA or are present inthe vector.

The ds cDNA is now ready to be used for insertion into a vector forexpression. As distinguished from the earlier vectors, which were solelyconcerned with replication of the ds cDNA, the vector which is employedat this stage requires the presence of the regulatory signals fortranscription and translation.

A vector is chosen having an appropriate promoter, as well as othertranscriptional regulatory signal sequences, such as an operator,attenuator, or activator. Also, the vector will have been at leastpartially sequenced, so as to determine the presence of at least oneinsertion site for introduction of the ds-cDNA coding for the variableregions at a site under the control of the regulatory signals.

Besides transcriptional regulatory signals there are, as alreadyindicated, translational regulatory signals, primarily the ribosomalbinding site (Shine-Dalgarno sequence, "S-D") and the initiation codon("f-met codon"). The S-D sequence and the initiation codon must be inthe proper spacing, generally spaced apart by from about 3 to 12 basepairs. The S-D sequence may be present on the vector in appropriatejuxtaposition to an insertion site or may be joined to the variableregion coding sequence, for example, by ligation of an oligonucleotideproviding the S-D sequence and an appropriate restriction site upstreamfrom the S-D sequence. Alternatively, the S-D sequence may be introducedby in vitro mutagenesis, as previously described. The coding sequencemust be in frame with the initiation codon.

In choosing the different strategies, considerations include thepresence or absence of particular restriction sites in the variableregion coding sequence and flanking regions; the availability of vectorswhich allow for insertion of the ds cDNA sequence into the vector andexpression of the variable region polypeptide; the availability ofuseful shuttle vectors; the availability of hosts which permitexpression and isolation in good yield; and the ability of the host torecognize such signals as secretory signals to cleave off the leadersequence. Therefore, in each situation with each different idiotype, itwill be necessary to restriction map at least portions of the DNAsequence coding for the variable region and the flanking regions.

Where the termini of the vector and sequence to be inserted are thesame, there will be the further concern that the inserted sequence maybe in the correct or incorrect orientation. By mapping the resultingcloned plasmids after insertion, one can select for those plasmidshaving the variable region sequence in the proper orientation.

The above strategy allows for a number of important advantages. Thepolypeptide chains are prepared as a homogeneous composition containingidentical sequences and chain lengths. The polypeptides forming the rFvwill be free of sugars. By virtue of the homogeneous and unglycosylatedcharacter of the polypeptides, the polypeptides may be more uniformlylabeled or modified. In this way products are obtained of uniform andreproducible properties. Thus, the products may be reliably administeredto a mammalian host without concern for unexpected responses due to aheterogeneous spectrum of products.

To recapitulate, in order to provide a homogeneous rFv having highbinding affinity, the evolutionary immune process is used as the focalpoint of the hybrid DNA strategy. The following steps are employed. Themessenger RNA from a hybridoma cell or other monoclonalantibody-producing cell is isolated and used to prepare a cDNAtranscript from the messenger encoding the light and/or heavy chains ofthe immunoglobulin. Based on the flanking sequences upstream anddownstream, at the initiation (may include leader region) andtermination of the variable region, short DNA sequences at leastpartially complementary to those sequences are employed for primerrepair or in vitro mutagenesis to remove extraneous flanking regions andto introduce translational control signals. The in vitro mutagenesisemploys an oligonucleotide, which heteroduplexes with one of the strandsof the cDNA, in combination with Klenow fragment of DNA polymerase I.Primer repair requires a homoduplexing oligonucleotide in combinationwith the same enzyme. The process is repeated twice to provide ds cDNAcoding for the variable region with translational regulatory signals atpredetermined sites. This ds cDNA is inserted into an appropriatevector, e.g. plasmid, to provide a DNA expression construct capable ofself-replication and having the proper regulatory signals forreplication, selection and expression.

The resulting construct is then introduced into an appropriate host toprovide expression of the heavy or light polypeptide members of the rFvand the polypeptides isolated. The heavy and light polypeptide membersof the rFv are then combined in an appropriate medium to form the rFv.

In view of the fact that the idiotypes vary, the sequence of steps ofthe subject invention permits the accommodation of a wide variety ofcoding sequences for variable regions. Also, the ds cDNA and vector canbe tailored to optimize the regulatory signals which are employed,particularly the promoter. The ribosome binding site and variable-regioninitiation codon may be properly spaced to optimize expression of thevariable-region polypeptide.

The constructs containing the variable region coding sequence in theproper orientation are used to transform the appropriate host forexpression. The resulting transformants are selected by virtue of themarkers present in the vector, cloned and expanded. The polypeptideproduced by the transformants may be isolated by separation of the cellsand isolation of the supernatant into which such polypeptides aresecreted. Or, if the polypeptides are not secreted, the transformantcells are isolated and lysed and the polypeptide recovered. Fractionscontaining enhanced amounts of the variable region polypeptide may beobtained by various conventional techniques, such as gelelectrophoresis, fractional precipitation, affinity chromatography, highpressure liquid chromatography, or the like. In any event, the originallysate, or supernatant, or the concentrated fractions therefrom, may bescreened for the presence of the variable-region polypeptides byimmunoassay.

Where the heavy and/or light chain is secreted, the chains may beisolated as follows. Polyclonal antisera to monoclonal immunoglobulincan be prepared by immunizing an appropriate vertebrate with the wholemonoclonal antibody, so as to produce antiserum which recognizes thedeterminant sites of the heavy and light chains. Antibodies recognizingthe whole immunoglobulin or only the light or heavy chain may besubstantially separated and purified from other antibodies in theantiserum. By binding to and eluting from affinity columns containingwhole immunoglobulin, or only the heavy or light chains, covalentlylinked to an appropriate support, the antibodies for the wholeimmunoglobulin, or heavy or light chain respectively, become bound tothe column. After denaturing the column and removing the purifiedantibodies, the antibodies are then conjugated to an appropriate supportto provide an affinity column to purify the heavy or light chains of therFv.

Where the light or heavy chain is not secreted, the transformedmicroorganisms containing the appropriate ds cDNA for either light orheavy chains are grown in liquid cultures and cleared lysates prepared.These lysates are then passed over an immunosorbent affinity columnprepared as described above, employing the specific polyclonal antisera.The bound variable regions are eluted from the column with anappropriate denaturing solvent. The eluates from each of the heavy andlight chain isolations are pooled, followed by treatment to renature thepolypeptides to form L-rFv and H-rFv respectively. For renaturation, thepooled eluates may be dialyzed against appropriate aqueous bufferedsolutions. The mixture is then further purified by passing over theappropriate ligand-affinity column and the bound molecules eluted withan appropriate denaturing solvent. The variable regions are thenrenatured as previously described to provide a solution of rFvs whichmay then be used for a variety of purposes.

In accordance with the subject invention, molecules are provided whichare polypeptide duplexes of the variable region of light and heavychains of immunoglobulins, retaining the specificity of theimmunoglobulins. By lacking the constant regions, the rFvs are lessimmunogenic and may, therefore, be prepared from sources xenogenic to ahost to which they are to be administered. Furthermore, the rFvs are ahomogeneous mixture, rather than a heterogeneous mixture. Theheterogeneous mixtures will contain chains of varying lengths, whichmixtures may be obtained by other techniques, such as enzyme and acidtreatment. The homogeneity of the compositions of the subject inventionallows for uniform modification and accurate determination oftherapeutic levels. In addition, there is no contamination with chainsfrom whole immunoglobulins which were inadequately digested, so as toretain immunogenic portions or uncover new immunogenic sites. Finally,large amounts of the desired rFvs may be prepared in high yield and highpurity.

The following examples are also by way of illustration and not by way oflimitation.

EXPERIMENTAL

Exemplary of various ligands, the following description will be directedto the dinitrophenyl ligand. It is to be understood that the subjectprocess will be useful for any ligand, although due to the wide varietyof idiotypes involved, at various stages the strategies may be requiredto be modified slightly to accommodate the presence of a particularrestriction site or other unique event.

EXAMPLE 1 Preparation of Monoclonal Antibodies for Dinitrophenyl

Into an aqueous buffered medium at about pH 10.5 is introduced 10 mmoles2,4-dinitrobenzene sulfonate and 0.01 mmole of keyhole limpet hemocyaninand the mixture rocked for 20 hours at room temperature. The solution isthen dialyzed against successive changes of 0.6 M NaCl and the residueisolated to be used for immunization.

The DNP immunogen (100 μg) is combined as an emulsion with 0.1 mlcomplete Freund's adjuvant and 0.1 ml PBS. To each of 6 BALB/c mice isinjected 0.2 ml of the above formulation. Each mouse receives fourinjections at weekly intervals. Each dose contains a total of 100 μg ofthe immunogen distributed intraperitoneally as well as subcutaneouslyinto foot pads and into inguinal areas. The first injection is givenwith complete and the remaining with incomplete Freund's adjuvant. Threedays after the last injection, the mice are sacrificed, the spleensisolated and used for formation of monoclonal antibodies.

The fusion is performed by combining 3×10⁷ Sp2/0-Ag14 myeloma cells(Shulman et al. (1978) Nature 276:269-270) and 5×10⁷ spleen cells andthe mixture centrifuged at 200 g for 5min and resuspended slowly in 0.6ml 50% PEG 1500 in Dulbecco's modified Eagle's medium (Flow). After 1min at 37° C., 20 ml of R medium (RPMI 1640 medium (Gibco) supplementedwith 30 mM Hepes (Flow)) is added slowly. The cells are then centrifugedand resuspended in 20 ml of R medium supplemented with 10% fetal calf'sserum (Gibco) (RF medium) and 0.2 ml of this suspension is thendistributed to each of 200 wells containing 0.8 ml RF medium. Onehundred of these wells also contain 2×10⁵ mouse peritoneal exudatecells. After 24h incubation, 1 ml RF supplemented with HAT medium isadded to each well. Every 2-3 days, 1 ml of the medium is replaced withfresh RF+HAT. After two weeks, the cells demonstrating growth are testedfor immunoglobulin production employing ³⁵S-2,4-dinitrophenylsulfenamide of lysine. Clones showing specificactivity are cloned by plating in soft agar to provide anti-DNP asrequired.

Alternatively, one may use the method described by Herzenberg et al.(1980) J. Exp. Med. 151: 1071-1087. In this method, DNP substitutedbovine serum albumin is added to individual wells in a microtiter platein an RIA diluent (1% BSA, 0.005 M EDTA and 0.1% NaN₃ in PBS pH7.6) (50μl, 0.05 mg/ml) and the mixture is incubated for 1 h in the wells. Testor standard antisera at various dilutions are then added to coated wells(20 μl/well) and incubated for 1 h. After washing three times with theRIA diluent, ¹²⁵ I-labeled anti-mouse immunoglobulin (approximately2×10⁵ cpm/well) is added and the mixture is incubated for 1 h. Platesare then washed 3× with the RIA diluent, dried and evaluated byautoradiography.

Both of these methods of detecting the presence of the desired antibodyare well known. The cells are then cloned either by limiting dilution orcloning in soft agar and the resulting cloned cell lines are propagatedand stored frozen in liquid nitrogen for use as required.

Cells from one of the positive cloned cell lines are grown to a densityof about 1×10⁶ cells/ml in a 1 L culture. The cells are harvested bycentrifugation and 1 gram of the cells is dropped into 16 ml ofguanidinium thiocyanate stock solution (4 M, 50 g of guanidiniumthiocyanate with 0.5 g of sodium N-lauryl sarcosine, 2.5 ml of 1 Msodium citrate, pH7.0, 0.7 ml of 2-mercaptoethanol and 0.5 ml of Sigma30% Antifoam A, and the volume brought to 100 ml at room temperature) ina 55 ml Potter-Elbehjem homogenizer tube and is immediately homogenizedfor 30-60s at full speed with an 18 mm diameter Tissumizer homogenizer(Tekmar Industries). The resulting homogenate is centrifuged for 10 minat 8,000 rpm in a Sorval HB4 swinging bucket rotor at 10° C. Thesupernatants are decanted into a flask, mixed with 0.024 volume(relative to the original volume of homogenizing buffer) of 1 M aceticacid to lower the pH from 7 to 5 and 0.75 volume of absolute ethanol.After capping and shaking the flask thoroughly, the flask is stored at-20° C. overnight and the material sedimented by centrifugation for 10min at -10° C. at 6,000 rpm in an HB4 rotor.

The resulting firm pellet is isolated, resuspended by vigorous shakingin 0.5 volume buffered guanidine hydrochloride stock solution (7.5 M,neutralized and then buffered with 0.25 volume of 1 M sodium citrate,pH7.0, 5 mM in dithiothreitol). The samples are briefly warmed in a 68°C. water bath to insure complete dispersion of the pellets, followed byprecipitation by adding 0.025 volume (relative to the amount ofguanidine hydrochloride) of 1 M acetic acid in 0.5 volume ethanol. Aftermaintaining the solution for at least 3 h at -20° C., the solution iscentrifuged and reprecipitated with guanidine hydrochloride asdescribed. The reprecipitated material is centrifuged for 5 min at 6,000rpm and thereafter all reactions are carried out under sterileconditions.

The final pellets are dispersed in ethanol at room temperature,triturated to extract excess guanidine hydrochloride and thencentrifuged for 5 min at 6,000 rpm. The ethanol is evaporated with astream of nitrogen and the RNA pellets dissolved with vigorous shakingin 1 ml of sterile water per g. of original cells. After centrifugationfor 10 min at 13,000 rpm at 10° C., the supernatant containing the RNAis decanted. To insure the complete extraction of all the RNA, theinsoluble material is reextracted twice with 0.5 ml of sterile water,the extract centrifuged for 10 min at 13,000 rpm at 10° C. and theaqueous solutions combined, mixed with 0.1 volume of 2 M potassiumacetate, pH5 and 2 volumes of ethanol and left overnight at -20° C.

The RNA is sedimented from the ethanol suspension by centrifugation for20 min at 20,000 rpm at -10° C. in Corex tubes in an HB4 rotor. Theresulting pellets are thoroughly washed with 95% ethanol, dried withnitrogen and dissolved in 1 ml/g cells of 10 mM Tris buffer pH 7.5, 1 mMEDTA, 0.2% SDS. After dissolution of the RNA pellet, 1/9 volume of 5 MNaCl is added, and the solution applied to an oligo(dT) column (about0.5 g dry weight, T3 grade, Collaborative Research). The column iswashed extensively with 0.5 M NaCl, 10 mM Tris, 1 mM EDTA, pH 7.5 0.2%SDS, and then eluted with 10 mM Tris, EDTA pH 7.5, 0.05% SDS. Theelution profile is monitored at A₂₆₀. The UV absorbing fractions arepooled and precipitated by addition of sodium acetate, pH 5 and 2.5volumes of ethanol. The dried pellet is dissolved in 50 μl (1 vol.) 10mM Tris 7.5 1 mM EDTA, and 9 vol. DMSO added, immediately followed by 1vol. of buffered 1 M LiCl (1 M LiCl, 50 mM EDTA, 2.0% SDS, 10 mM Tris,pH 6.5). This solution is heated at 55° for 5 min, 100 vol. of bindingbuffer added, and then reapplied to the oligo(dT) cellulose column,equilibrated with binding (0.5 M NaCl, 10 mM Tris, 1 mM EDTA, .2% SDS)buffer and eluted as before.

The presence of messenger RNA encoding the monoclonal immunoglobulinheavy and light chain polypeptides is verified by hybrid selectionemploying DNA clones of the appropriate heavy and light chain genes fromsources described in Early and Hood, Genetic Engineering (1981) Vol. 3,Setlow and Hollander, Plenum Publishing Corp., pages 157-188. DNA probescan be prepared by synthesis, based on published amino acid sequences orpublished DNA sequences or obtained from a variety of sources reportedin Early and Hood, supra. The DNA probes are denatured, neutralized andbound to nitrocellulose filter paper (Schleicher and Schuell BA-85-R597) according to the method of Southern, J. Mol. Biol. (1975)98:503-517, in 10×conc. standard citrate. (See also, U.S. Pat. No.4,302,204.) The probes are hybridized to 30 μg of the messenger RNA in65% formamide/10 mM Pipes, pH6.4/0.4 M NaCl in a final volume of 100 μlat 50° C. for 2 h. The reaction mixture is spun for 10 sec. in aMicrofuge, vortexed, spun again and then gently vortexed to resuspendthe filters. The mixture is incubated at 50° C. for about 1 h with mildagitation. The reaction mixture is then removed and the filters arewashed in 1 ml 0.15 M NaCl/0.015 M Na citrate/0.5% NaDodSO₄ 10x, whilemaintaining the wash buffer at 60° C. After each addition of washbuffer, the tubes are vortexed for several seconds. The filters are thenwashed twice with 1 ml 10 mM Tris, pH 7.8, 2 mM EDTA, the tubes beingincubated at 60° C. for 5 min and the solution removed by aspiration.

RNA is eluted from the RNA-DNA hybrid by boiling the filters for 60 secin 300 μl of double distilled, sterile water and then quick-frozen in amethanol/dry ice bath. The liquid is removed and brought to a finalconcentration of 0.2 M of sodium acetate and 20 μg of calf thymus tRNAis added. The RNA is precipitated with 2.5 volume of ethanol at -20° C.and immediately prior to translation the RNA is pelleted at 12,000 g for10 min at 4° C., the pellet washed twice with 70% ethanol and then driedunder reduced pressure.

The eluted mRNA is now translated in vitro with rabbit reticulocytecell-free extract. A translation kit, such as the commercially availablekit from New England Nuclear may be employed. After translation, thepresence of protein synthesis is determined in accordance with theinstructions of the supplier.

After establishing the presence of translation of messenger RNA,aliquots are taken and incubated with monoclonal antibodies insubstantial excess to the amount of expression product in the lysatecomposition. The complex is then precipitated with S. aureus and theprecipitates are washed 3×in 0.05 M tris, pH 8.3, 0.45 M NaCl in 0.5%NP40, boiled in 0.01 M sodium phosphate buffer, pH7.5, containing 1%β-mercaptoethanol and electrophoresed on 5-20% gradientSDS-polyacrylamide gels. The gels are run at 125 V for 1 h after thebromophenol blue marker runs off the end of the gel. The gels are thendried, fixed and autoradiographed on Kodak X-R film.

Having extablished the presence of messenger RNA coding forimmunoglobulin light and heavy chains, the messenger RNA mixture is thenemployed to prepare a library of double stranded cDNA employing themethod of Okayama and Berg, supra. Four hundred μg of pBR322-SV40(0.71-0.86) DNA are digested at 37° with 700 units of KpnI endonucleasein a reaction mixture (0.4 ml) containing 5 mM tris-HCl (pH 7.5), 6 mMMgCl₂, 6 mM NaCl, 6 mM 2-mercaptoethanol and 0.1 mg/ml bovine serumalbumin (BSA). After 5 hrs, the digestion is terminated with 40 μl of0.25 M EDTA (pH 8.0) and 20 μl of 10% SDS; the DNA is recoveredfollowing extraction with water saturated phenol--CHCl₃ (1:1) (hereafterreferred to as phenol--CHCl₃) and ethanol precipitation.

Homopolymer tails averaging 60, but not more than about 80, dT residuesper end are added to the KpnI endo-nuclease-generated termini with calfthymus terminal deoxynucleotidyl transferase as follows: The reactionmixture (0.2 ml) contains as buffer 140 mM sodium cacodylate-30 mMtris-HCl (pH 6.8), 1 mM CoCl₂, 0.1 mM dithiothreitol, 0.25 mM dTTP, theKpnI endonuclease-digested DNA and 400 units of the terminaldeoxynucleotidyl transferase. After 30 minutes at 37° C. the reaction isstopped with 20 μl of 0.25 M EDTA (pH 8.0) and 10 μl of 10% SDS and theDNA is recovered after several extractions with phenol--CHCl₃ by ethanolprecipitation. The DNA is then digested with 17 units of HpaIendonuclease in 0.2 ml containing 10 mM Tris-HCl (pH 7.4), 10 mM MgCl₂,20 mM KCl, 1 mM dithiothreitol and 0.1 mg/ml BSA for 5 hrs at 37° C.

The large DNA fragment, which contains the origin of pBR322 DNAreplication and the gene conferring ampicillin resistance, is purifiedby agarose (1%) gel electrophoresis and is recovered from the gel by amodification of the glass powder method (Vogelstein and Gillespie, PNASUSA (1979) 76:615-619).

The dT-tailed DNA is further purified by adsorption and elution from anoligo dA-cellulose column as follows: The DNA is dissolved in 1 ml of 10mM tris-HCl (pH 7.3) buffer containing 1 mM EDTA and 1 M NaCl, cooled to0° and applied to an oligo dA-cellulose column (0.6×2.5 cm) equilibratedwith the same buffer at 0°. The column is washed with the same buffer at0° and eluted with water at room temperature. The eluted DNA (140 μg) isprecipitated with ethanol and dissolved in 100 μl of 10 mM Tris-HCl (pH7.3) with 1 mM EDTA.

The oligo dG-tailed linker DNA is prepared by digesting 100 μg ofpBR322-SV40 (0.19-0.32) with 120 units of PstI endonuclease in 0.2mlcontaining 6mM Tris-HCl (pH 7.4), 6 mM MgCl₂, 6 mM 2-mercaptoethanol, 50mM NaCl and 0.1 mg/ml BSA. After 1.5 hrs at 37° the reaction mixture isextracted with phenol-CHCl₃ and the DNA is precipitated with alcohol.Then, tails of 10-15 dG residues are added per end with 60 units ofterminal deoxynucleotidyl transferase in the same reaction mixture (50μl ) described above, except for 0.1 mM dGTP replacing dTTP. After 20minutes at 37° C. the mixture is extracted with phenol--CHCl₃ and afterthe DNA is precipitated with ethanol it is digested with 50 units ofHindIII endonuclease in 50 μl containing 20 mM Tris-HCl (pH 7.4), 7 mMMgCl.sub. 2, 60 mM NaCl and 0.1 mg/ml BSA at 37° for 1 hr. The smalloligo dG-tailed linker DNA is purified by agarose (1.8%) electrophoresisand recovered as described above.

The reaction mixture (10 μl ) contains 50 mM Tris-HCl (pH 8.3), 8 mMMgCl₂, 30 mM KCl, 0.3 mM dithiothreitol, 2 mM each dATP, dTTP, dGTP, and³² P-dCTP (850 cpm/pmol), 0.2 μg of the mRNA (about 2-3 fold excess overprimer ends), 1.4 μg of the vector-primer DNA (0.7 pmole primer end) and5 units of reverse transcriptase. (The molar ratio of polyA mRNA tovector-primer DNA ranges from about 1.5-3).

cDNA synthesis is initiated by the addition of reverse transcriptase andcontinued at 37° for 20 min. By this time the rate of dCTP incorporationlevels off and more than 60% of the primer is utilized for cDNAsynthesis. The reaction is stopped with 1 μl of 0.25 M EDTA (pH 8.0) and0.5 μl of 10% SDS; 10 μl of phenol-CHCl₃ is added and the solutionvortexed vigorously and then centrifuged. After adding 10 μl of 4 Mammonium acetate and 40 μl of ethanol to the aqueous phase, the solutionis chilled with dry ice for 15 min, warmed to room temperature withgentle shaking to dissolve unreacted deoxynucleoside triphosphates thatprecipitate during chilling, and centrifuged for 10 min in an Eppendorfmicrofuge. The pellet is dissolved in 10 μl of 0 mM Tris-HCl (pH 7.3)and 1 mM EDTA, mixed with 10 μl of 4 M ammonium acetate andreprecipitated with 40 μl of ethanol, and then rinsed with ethanol.

The pellet containing the cDNA:mRNA- plasmid is dissolved in 15 μl of140 mM sodium cacodylate-30 mM Tris-HCl (pH 6.8) buffer containing 1 mMCoCl₂, 0.1 mM dithiothreitol, 0.2 μg of poly A, 66 μM ³² P-dCTP (6000cpm/pmol) and 18 units of terminal deoxynucleotidyl transferase. Thereaction is carried out at 37° for 5 min to permit the addition of 10 to5 residues of dCMP per end and then terminated with 1.5 μl of 0.25 MEDTA (pH 8.0) and 0.75 μl of 10% SDS. After extraction with 15 μl ofphenol-CHCl₃ the aqueous phase is mixed with 15 μl of 4 M ammoniumacetate and the DNA is precipitated and reprecipitated with 60 μl ofethanol and the final pellet rinsed with ethanol.

The pellet is dissolved in 10 μl of buffer containing 20 mM Tris-HCl (pH7.4), 7 mM MgCl₂ 60 mM NaCl and 0.1 mg/ml BSA and then digested with 2.5units of HindIII endonuclease for 1 hr at 37° . The reaction isterminated with 1 μl of 0.25 M EDTA (pH 8.0) and 0.5 μl of 10% SDS and,after extraction with phenol-CHCl₃, followed by the addition of 10 μl of4 M ammonium acetate, the DNA is precipitated with 40 μl of ethanol. Thepellet is rinsed with ethanol, dissolved in 10 μl of 10 mM Tris-HCl (pH7.3) and 1 mM EDTA and 3 μl of ethanol are added to prevent freezingduring storage at -20° C.

One μl of the HindIII endonuclease-digested oligo dC-tailedcDNA:mRNA-plasmid (0.02 pmol) is incubated in a mixture (10 μl )containing 10 mM Tris-HCl (pH 7.5) 1 mM EDTA, 0.1 M NaCl and 0.04 pmolof the oligo dG-tailed linker DNA (this amount is a two-fold molarexcess over the quantity of the vector-cDNA:mRNA and of the fragmentwhich remains as a result of the HindIII endonuclease digestion in theprevious step) at 65° for 2 min., followed by 42° for 30 min. and thencooled at 0°. The mixture (10 μl ) is adjusted to a volume of 100 μlcontaining 20 mM Tris-HCl (pH 7.5), 4 mM MgCl₂, 10 mM (NH₄)₂ SO₄ 0.1 MKCl, 50 μg/ml BSA and 0.1 mM β-NAD; after adding 0.6 μg of E. coli DNAligase the solution is incubated overnight at 12°.

To replace the RNA strand of the insert, the ligation mixture isadjusted to contain 40 μM of each of the four deoxynucleotidetriphosphates, 0.15 mM β-NAD, 0.4 μg of additional E. coli DNA ligase,0.3 μg of E. coli DNA polymerase I, and 1 unit of E. coli RNase H. Thismixture (104 μl ) is incubated successively at 12° and room temperaturefor 1 hr each to promote optimal repair synthesis and nick translationby PolI. The reaction is terminated by the addition of 0.9 ml of cold 10mM Tris-HCl (pH 7.3) and 0.1 ml aliquots are stored at 0°.

Transformation is carried out using minor modifications of the proceduredescribed by Cohen et al., PNAS USA (1972) 69:2110-2114. E. coli K 12(strain HB101) is grown to 0.5 A₆₀₀ at 37° C. in 20 ml L-broth. Thecells are collected by centrifugation, suspended in 10 ml of 10 mMTris-HCl (pH 7.3) containing 50 mM CaCl₂ and centrifuged at 0° for 5min. The cells are resuspended in 2 ml of the above buffer, incubatedagain at 0° for 5 min.; then, 0.2 ml of the cell suspensions is mixedwith 0.1 ml of the DNA solution and incubated at 0° for 15 min. Afterthe cells are kept at 37° for 2 min. and at room temperature for 10min., 0.5 ml of L-broth is added, the culture incubated at 37° for 30min, and then plated on nitrocellulose filters on agar plates containing50 μg/ml ampicillin. After incubation at 37° for 12-24 hrs. E. colitransformants are screened for the presence of the light and heavy chaincDNA according to the method of Grunstein and Hogness by in situ colonyhybridization. Several thousand transformants are grown on three replicanitrocellulose filter discs, lysed with alkali and hybridized with theprobes described previously for the constant regions of the heavy andlight immunoglobulin chains. Clones of the genes coding for the heavyand light immunoglobulin chains are identified. Colonies that givepositive hybridization signals are grown in one-liter of L-brothcontaining 50 μg/ml of ampicillin and their plasmid DNAs are isolated bystandard techniques (Gunsalus et al., J. Bact. (1979) 140:106-113).

The cells are lysed as described previously, the lysate cleared bycentrifugation and the cleared lysate diluted with an equal volume ofwater. RNase A is added to 50 μg/ml and after 1 h at 37° C., the lysateis extracted with 0.3 volume of phenol saturated with TE buffer (10 mMtris-HCl, pH 7.9, plus 1 mM EDTA). After centrifugation (16,000×g, 4°C., 10 min), the aqueous phase is removed, adjusted to 1 M NaCl and theDNA precipitated with 2 volumes of ethanol. After several hours at -20°C., the DNA is pelleted by centrifugation (10,000×g, 4° C., 20 min),dried and dissolved in TE buffer.

Each of the cDNA clones are then restriction mapped and sequenceanalyzed by conventional techniques, so that a restriction map isobtained which allows for subsequent manipulation of the cDNA coding forthe variable regions for cloning and expression. The methods of Maxamand Gilbert, Methods Enzymol. (1980) 65:499-560 and Sanger et al., J.Mol. Biol. (1980) 143:161-178 are used, respectively. Those cDNA clonesfor light chains and heavy chains encoding the complete variable regionand leader sequences are selected for subsequent manipulation.

Illustrative of the subject method will be the isolation, sequencing andmanipulation of the κ-chain (light chain) of MOPC41 and the heavy chainof the myeloma S107.

The following is the sequence of the k-chain of MOPC41, where thesequences encoding the leader, variable region and constant region areseparated by gaps, with only the first sixteen amino acids of theconstant region indicated. (Seidman et al.,"Nature" (1979) 280: 370-375)

    __________________________________________________________________________                        Met Asp Met Arg Ala Pro Ala                               . . .                                                                             TCA GGA CTC AGC ATG GAC ATG AGG GCT CCT GCA                               Gln Ile Phe Gly Phe Leu Leu Leu Leu Phe Gln Gly                               CAG ATT TTT GGC TTC TTG TTG CTC TTG TTT CAA GGT                               Thr Arg Cys     Asp Ile Gln Met Thr Gln Ser Pro                               ACC AGA TGT . . .                                                                             GAC ATC CAG ATG ACC CAG TCT CCA                               Ser Ser Leu Ser Ala Ser Leu Gly Glu Arg Val Ser                               TCC TCC TTA TCT GCC TCT CTG GGA GAA AGA GTC AGT                               Leu Thr Cys Arg Ala Ser Gln Asp Ile Gly Ser Ser                               CTC ACT TGT CGG CCA AGT CAG GAC ATT GGT AGT AGC                               Leu Asn Trp Leu Gln Gln Glu Pro Asp Gly Thr Ile                               TTA AAC TGG CTT CAG CAG GAA CCA GAT GGA ACT ATT                               Lys Arg Leu Ile Tyr Ala Thr Ser Ser Leu Asp Ser                               AAA CGC CTG ATC TAC GCC ACA TCC AGT TTA GAT TCT                               Gly Val Pro Lys Arg Phe Ser Gly Ser Arg Ser Gly                               GGT GTC CCC AAA AGG TTC AGT GGC AGT AGG TCT GGG                               Ser Asp Tyr Ser Leu Thr Ile Ser Ser Leu Glu Ser                               TCA GAT TAT TCT CTC ACC ATC AGC AGC CTT GAG TCT                               Glu Asp Phe Val Asp Tyr Tyr Cys Leu Gln Tyr Ala                               GAA GAT TTT GTA GAC TAT TAC TGT CTA CAA TAT GCT                               Ser Ser Pro Trp Thr Phe Gly Gly Gly Thr Lys Leu                               AGT TCT CCG TGG ACG TTC GGT GGA GGC ACC AAG CTG                               Glu Ile Lys Arg     Ala Asp Ala Ala Pro Thr Val                               GAA ATC AAA CGT . . .                                                                             GCT GAT GCT GCA CCA ACT GTA                               Ser Ile Phe Pro Pro Ser Ser Glu Gln                                           TCC ATC TTC CCA CCA TCC AGT GAG CAG . . .                                     __________________________________________________________________________

The following is the nucleotide sequence of the heavy chain variableregion of myeloma S107, with the leader, variable region and constantregion separated by gaps, and only the first nine amino acids of theconstant region depicted. (Early et al. (1980), Cell. 19:981-992).

    __________________________________________________________________________    Met Lys Leu Trp Leu Asn Trp Val Phe Leu Leu Thr Leu                           ATG AAG TTG TGG TTA AAC TGG GTT TTT CTT TTA ACA CTT                           Leu His Gly Ile Gln Cys . . .                                                                             Glu Val Lys Leu Val Glu                           TTA CAT GGT ATC CAG TGT     GAG GTG AAG CTG GTG GAA                           Ser Gly Gly Gly Leu Val Gln Pro Gly Gly Ser Leu Arg                           TCT GGA GGA GGC TTG GTA CAG CCT GGG GGT TCT CTG AGA                           Leu Ser Cys Ala Thr Ser Gly Phe Thr Phe Ser Asp Phe                           CTC TCC TGT GCA ACT TCT GGG TTC ACC TTC AGT GAT TTC                           Tyr Met Glu Trp Val Arg Gln Pro Pro Gly Lys Arg Leu                           TAC ATG GAG TGG GTC CGC CAG CCT CCA GGG AAG AGA CTG                           Glu Trp Ile Ala Ala Ser Arg Asn Lys Ala Asn Asp Tyr                           GAG TGG ATT GCT GCA AGT AGA AAC AAA GCT AAT GAT TAT                           Thr Thr Glu Tyr Ser Ala Ser Val Lys Gly Arg Phe Ile                           ACA ACA GAG TAC AGT GCA TCT GTG AAG GGT CGG TTC ATC                           Val Ser Arg Asp Thr Ser Gln Ser Ile Leu Tyr Leu Gln                           GTC TCC AGA GAC ACT TCC CAA AGC ATC CTC TAC CTT CAG                           Met Asn Ala Leu Arg Ala Glu Asp Thr Ala Ile Tyr Tyr                           ATG AAT GCC CTG AGA GCT GAG GAC ACT GCC ATT TAT TAC                           Cys Ala Arg Asp Tyr Tyr Gly Ser Ser Tyr Trp Tyr Phe                           TGT GCA AGA GAT TAC TAC GGT AGT AGC TAC TGG TAC TTC                           Asp Val Trp Gly Ala Gly Thr Thr Val Thr Val Ser Ser                           GAT GTC TGG GGC GCA GGG ACC ACG GTC ACC GTC TCC TCA                               Ala Lys Thr Thr Pro Pro Thr Val Tyr                                       . . .                                                                             GCC AAA ACG ACA CCC CCA TCT GTC TAT . . .                                 __________________________________________________________________________

Based on the DNA sequencing and the restriction map, PstI sites arefound at the -110 base pair of the coding strand and downstream from thetermination site for the cDNA coding for the light chain, whileconvenient Hind III restriction sites are found upstream from the leadersequence and downstream from the termination site of the coding strandfor the heavy chain. The leader sequences and coding sequences of thelight and heavy chain variable regions are free of sequences recognizedby the indicated endonucleases.

The isolated plasmid DNAs are digested with the respective endonucleasesin accordance with the instructions of the supplier and the resultingfragments purified by electrophoresis on agarose gels (Seakem). The gelsare 2% agarose, 15 cm ×15 cm×0.2 cm and 100 V for 2 h is applied. Byemploying markers, the band of the appropriate molecular weight islocated and excised. The gel slice is placed directly into an 1.5 mlEppendorf tube, rapidly frozen and thawed twice in a Dry Ice-alcoholbath and then centrifuged 5 min in the Eppendorf centrifuge (15,000 rpm)and the supernatant recovered. The supernatant is boiled in 6×SSC todenature the DNA and provide single strands, followed by cooling to 0°C.

Based on the DNA sequence, a DNA oligomer is prepared which is at leastpartially complementary to a short sequence of each of the non-coding("anti-sense") strands of the variable region sequences of the light andheavy chains. The oligomer has an f-met codon at its 5'-end and iscomplementary to the downstream nucleotides at the N-terminus of theleader sequence for primer repair: or has an f-met codon intermediateits ends and complementary sequences to the 3'-end of the codingsequence for the leader region and the 5'-end of the coding sequence forthe variable regions for in vitro mutagenesis. The oligomers are readilyprepared in accordance with the methods described by Itakura et al. J.Biol. Chem. (1975) 150:4592.

The following schemes depict the primer repair synthesis method for thelight and heavy chains where the leader sequence is retained (a and b,respectively) and the in vitro mutagenesis method where the leadersequence is removed and an f-met codon introduced at the N-terminus ofthe coding sequence for the variable regions of the light and heavychains (c and d, respectively).

    ______________________________________                                        ATG GAC ATG AGG GCT - 3'                                                           ##STR1##                                                                     ATG GAC ATG AGG GC                                                             ##STR2##                                                                 ATG GAC ATG AGG GC - 3'                                                       TAC CTG TAC TCC CG - 5'                                                       ATG AAG TTG TGG - 3'                                                               ##STR3##                                                                     ATG AAG TTG TGG                                                                ##STR4##                                                                 ATG AAG TTG TGG - 3'                                                          TAC TTC AAC ACC - 5'                                                          GGT ACC AGA TGT GAC ATC CAG - 3'                                                   ##STR5##                                                                     GG TAC CAG ATG GAC ATC C                                                       ##STR6##                                                                 GG TAC CAG ATG GAC ATC C - 3'                                                      ##STR7##                                                                 GG TAC CAG ATG GAC ATC C - 3'                                                 CC ATG GTC TAC CTG TAG G - 5'                                                 GGT ATC CAG TGT GAG GTG - 3'                                                       ##STR8##                                                                      ##STR9##                                                                      ##STR10##                                                                     ##STR11##                                                                     ##STR12##                                                                GGT ATC CAG ATG GAG GTG - 3'                                                  CCA TAG GTC TAC CTC CAC - 5'                                                  ______________________________________                                    

To 0.5 μg of the single stranded DNA is added 15 pmole of5'-phosphorylated oligonucleotide as described in a and b above in 38 μlof 200 mM of NaCl, 13 mM tris-HCl, pH 7.5, 9 mM Mg acetate, 20 mMβ-mercaptoethanol, the mixture boiled for 3 min and immediately cooledto 0° C. To this is added 1 μl of solution which contains the fourdeoxynucleoside triphosphates at 4 mM, 0.1 μl of 100 mM adenosinetriphosphate, and 1 μl (1 unit) of the Klenow fragment of DNA polymeraseI (Boehringer Mannheim).

In this manner, strands coding for the 5'-leader sequence and codingsequence or just the coding sequence for the variable region aresynthesized and the single-stranded DNA sequences in the 3'-direction ofthe template non-coding strand are degraded by the 3'-5'-exonucleaseactivity. As a result, for strands containing the leader sequence,homoduplexes are obtained for coding the leader sequence and variableregions for both the light and heavy chains which are blunt ended,having an initiation codon at the 5'-end of the coding strand with theremaining DNA sequence in frame with the initiation codon.

To the resulting blunt ended duplex coding for the leader sequence andvariable region of the chains, restriction enzyme linkers are ligatedthrough the use of appropriate phosphorylated linkers, for example, PstIlinkers, employing T4 polynucleotide ligase under conditions specifiedby the supplier. The vector pBR322 is cleaved with PstI to providecohesive ends for linking to the modified cDNA.

Each of the cDNAs are combined with the linear pBR322 havingcomplementary termini. Equal molar amounts of the vector and cDNAs arecombined in an annealing buffer essentially as described in Steinmetz etal. (1981) Cell. 24:125-134, and the annealed DNA used directly fortransformation.

One ml of an overnight bacterial culture E. coli strain HB101 (Boyer andRoulland-Dussiox (1969) J. Mol. Biol. 41:459-472) is grown to 2×10⁸cells/ml in L broth, pelleted by centrifugation (Sorval SS34 rotor,85,000 rpm, 4° C., 5 min) and washed in 0.5 volume cold 10 mM CaCl₂. Thecell pellet is resuspended in 0.5 volume cold 30 mM CaCl₂. After 20-minon ice, the cells are again pelleted and resuspended in 0.1 volume cold30 mM CaCl₂. Then 0.20 ml of the suspension is added to 0.1 ml 30 mMCaCl₂ containing the annealed plasmids and incubated on ice for 16 min.Each transformation is then heated to 42° C. for 75 sec prior to theaddition of 5 ml L broth.

Transformed cultures are incubated at 37° C. for 2 hr. The transformantsare then grown in agar plates containing M-9 minimal medium and 10 μg/mltetracycline. Clones which grow on this medium are then transferred toagar plates having M-9 minimal medium and 40 μg/ml of ampicillin. Thosecells which are sensitive to ampicillin and resistant to tetracyclineare then screened for the presence of plasmids having the desired cDNA.

The selected clones are then grown in 2 ml of nutrient culture for 18 h.A 0.5 ml aliquot is transferred to a 1.5 ml Eppendorf tube for plasmidextraction. Manipulations are carried out at room temperature unlessotherwise indicated. The tube is centrifuged for 15 sec, the supernatantcarefully removed with a fine-tip aspirator and the cell pellet isthoroughly suspended in 100 μl of a lysozyme solution containing 2 mg/mllysozyme 50 mM glucose, 10 mM EDTA, 25 mM tris-HCl (pH8.0).

After a 30 min incubation at 0 ° C., 200 μl of alkaline SDS solution(0.2 N NaOH, 1% sodium dodecylsulfate) is added and the tube is gentlyvortexed. The tube is maintained for 5 min at 0° C. and then 150 μl of 3M sodium acetate (pH4.8) is added. After gently mixing by inversion fora few seconds, a clot of DNA forms and the tube is maintained at 0° C.for 16 min. After centrifugation for 5 min, 0.4 ml of the supernatant isremoved, transferred to a second centrifuge tube, 1 ml cold ethanoladded and the tube held at -20° C. for 30 min. The precipitate iscollected by centrifugation for 2 min and the supernatant removed byaspiration. The pellet is resuspended in 100 μl 0.1 M sodium acetate,200 μl ethanol added, and after 10 min at - 20° C., the precipitate isagain collected by centrifugation, and the pellet is dissolved in 50 μlwater.

Substantially, the same procedure as described above is used for invitro mutagenesis. With the primer repair synthesis, only one homoduplexis formed; with in vitro mutagenesis, a heteroduplex is initially formedwhich upon transformation and cloning results in two homoduplexes: theoriginal gene sequence; and the modified or "tailored" gene sequence,which includes the change in sequence encoded in the oligomer.

As depicted in c and d, oligomers are prepared which introduce aninitiation (f-met) codon at the N-terminus of the coding sequence forthe variable regions.

The resulting plasmid DNA is isolated as described above and used againas described above for transformation. However, the resultingtransformants are grown in small (2 ml) culture for plasmid isolation.The plasmid DNA prepared from single transformant colonies arising fromthe second cycle of cloning are assayed by filter blot hybridization onnitrocellulose filters (Wallace et al. (1979) Nucleic Acids Research6:3543-3556) probing with ³² P-radiolabeled oligomers employed for themutagenesis so as to insure the isolation of the desired tailoredhomoduplexes of the cDNA. The clones having the tailored sequence areisolated and the plasmid DNA extracted for further processing at the3'-end of the coding strand.

The cDNA coding for the variable regions can be excised by digestionwith Pstl. Repeating the technique described in the previous in vitromutagenesis, where an ATG ("start") codon is introduced before the codonof the N-terminal amino acid of the mature polypeptide, "stop" codonsare introduced at the C-terminus of the variable regions.Oligonucleotides are prepared as described previously havingcomplementary sequences to the coding ("sense") strand of thevariable-region cDNA.

The oligonucleotides and the schemes for inserting the stop codon at theend of the variable regions are depicted as follows. The introduction ofthe stop codon in the κ light chain is set forth in e, while theintroduction of the stop codon in the heavy chain is set forth in f.

    ______________________________________                                        GAA ATC AAA CGT GCT GAT GCT GCA CC - 3'                                            ##STR13##                                                                     ##STR14##                                                                     ##STR15##                                                                     ##STR16##                                                                     ##STR17##                                                                AAA CGT TGA TGC TGC ACC - 3'                                                  TTT GCA ACT ACG ACG TGG - 5'                                                  ACC GTC TCC TCA GCC AAA ACG ACA - 3'                                               ##STR18##                                                                     ##STR19##                                                                     ##STR20##                                                                     ##STR21##                                                                     ##STR22##                                                                    5' C TCC TCA TGA AAA ACG CA - 3'                                          G AGG AGT ACT TTT TGC TGT - 5'                                                ______________________________________                                    

With the replication of the coding strand extending the oligomer havingthe stop codon, there is also exonuclease activity of the polymerasewhich degrades the coding strand, removing all of the sequence codingfor the constant region, except for the few nucleotides present in theoligonucleotide.

The heteroduplexes having the "tailored" sequences of the variableregions of the light and heavy chains are then ligated to PstI linkers,restricted with PstI endonuclease and inserted into the PstI site ofpBR322. After cloning and recloning, the plasmids containing thetailored ds cDNA with the stop codons at the end of the variable regionsare isolated and the sequences coding for the variable regions (whichmay also include the leader sequences) are excised from the pBR322plasmid using the PstI restriction endonuclease and may now be used forexpression of the polypeptide chains of the rFv.

In order to obtain expression of the variable regions, the plasmid pGMl(pVH253ΔtrpLE1413; Miozarri and Yanofsky, J. of Bacteriol. (1978)133:1457-1466) is employed. The plasmid is modified to introduce a PstIsite which provides for insertion of the sequences coding for thevariable regions with the f-met codon in proper position to theShine-Dalgarno sequence. The following oligonucleotide sequence isprepared:

AGCTGCAGCTTTCGTT.

pGMl(10 μg) is nicked in one strand by digestion with EcoRI (BoehringerMannheim, 1000 units) in 1 ml of 100 mM tris-HCl, pH 7.2, 50 mM NaCl, 5mM Mg acetate, 0.01 percent NP-40 and 150 μg/ml ethidium bromide at 37°C. for 1 h. After bringing the reaction mixture to 10 mM EDTA, it isextracted 3×10 volumes water-saturated isobutanol, 1 ×phenol-CHCl₃,2×ether and 1 ×isobutanol to reduce the volume to 0.1 ml. Afterdesalting by centrifugation through a 0.5 ml Sephadex G-25 column, theDNA is recovered by precipitation with ethanol. Approximately 5 μg ofthe nicked DNA is incubated with 40 units of exonuclease III (BRL) in20μl of 10 mM tris-HCl, pH 7.5, 2 mM MgCl₂ and 1 mM β-mercaptoethanolfor 90 min at 37° C. The reaction is adjusted to 15 mM tris-HCl, pH 7.5,7 mM NaCl, 7 mM MgCl₂, 7 mM dithiothreitol. After adding 20 units ofbacterial alkaline phosphatase (BRL) and 5 units of HinfI (BRL),digestion is continued for 30 min at 37° C. The mixture is brought to 10mM EDTA, extracted 2×phenol-CHCl₃, 1×ether and desalted bycentrifugation through 0.5 ml Sephadex G-25 equilibrated with water.

A major portion of the resulting circular ssDNA is combined with 50pmole of the 5 '-phosphorylated oligonucleotide, depicted above forintroducing the PstI site, in 38 μl of 200 mM NaCl, 13 mM tris-HCl, pH7.5, 9 mM magnesium acetate, 20 mMβ-mercaptoethanol, boiled for 30 minand immediately cooled to 0° C. After adding 5 μl of a solution 4 mM inthe four dXTP, 0.5 μl of 100 mM ATP, 3 μl (3 units) of DNA polymerase I(Klenow fragment) and 4 μl (10 units) of T4 DNA ligase, the mixture isincubated overnight at 12° C. and then used directly for transformationof E. coli HB101 and the transformants grown, isolated and analyzedusing blot hybridization employing radiolabeled ³² P-oligomer to detectclones having the tailored sequence containing the new PstI site.

The "tailored" pGM1 is isolated, partially restricted with PstI and theDNA sequences coding for the light and heavy chain variable regionsprepared above inserted individually into the tailored site to providetwo plasmids having DNA sequences coding for the light (pGM1L) and heavy(pGM1H) chains, in accordance with the procedure described previouslyfor insertion. The resulting plasmids are used to transform E. coliHB101 and clones having the light and heavy variable region sequences inthe desired orientation identified by restriction mapping and purified.

Antisera recognizing the light and heavy chains respectively areproduced by using the particular chains as immunogens and the antiseraisolated and covalently linked to Sepharose by conventional procedures(March et al., Anal. Biochem. (1974) 60:149-152) and the productsemployed for affinity columns.

The transformants are grown to cell densities of about 10⁹ cells/ml andcollected by centrifugation. The pellet is resuspended in 50 μl of 50 mMtris-HCl, pH8, 50 mM EDTA, 15% sucrose, 1 mg/ml lysozyme, 0.5% NP40.After 30 min at 0° C., 10 μl of 150 mM tris-HCl, pH 7.5, 280 mM MgCl₂, 4mM CaCl₂ and 1 μg DNase are added, followed by centrifugation for 15 minat 12,000 g.

The protein is then isolated by removal of the supernatant from thepellet and the supernatants are passed over the immunosorbent columns(0.15 ml) equilibrated with tris-HCl, pH 7.5. The light and heavy chainsof the rFv are eluted with 1 M acetic acid, pH 2.5 and the eluatespooled and neutralized with 0.1 M NaOH at 0° C. to pH 5.5. The pooledeluates are dialyzed against 3×100 volumes of sodium acetate buffer, pH5.5, followed by 3×100 volumes PBS, pH 7.

The renatured light and heavy chains of the rFv are further purified bycombining the eluates containing the rFv components and passing themover a DNP-affinity column. (In the present example, different sourcesof heavy and light chains are described, so that this step is done wherethe source of the two chains is the same.) A DNP-affinity column andprocedure is described in Kooistra and Richards, Biochem. (1978)17:345-351. In addition, sulfhydryl groups may be capped withiodoacetamide as described by Kooistra and Richards, ibid.

The bound rFv is isolated by elution with 1 M acetic acid, followed byrenaturing with sequential dialysis as described above.

The subject method provides protein complexes of homogeneous compositionhaving two peptide chains which form a complex having high bindingaffinity for a predetermined haptenic site. The two chains form an rFvhaving specificity for a particular ligand, by mimicking a naturallyoccurring immunoglobulin. By removing the constant regions, theresulting rFv has reduced immunogenicity and lacks peptide sequenceswhich may have undesirable functions for particular applications e.g.complement fixation.

The rFv can be used for a variety of purposes in diagnosis and therapy.Because of the homogeneous nature of the composition, the compositionhas a fixed reproducible level of immunogenicity. Also, due to thereduced molecular weight, relatively short residence times will beinvolved after injection into a mammalian host. This is particularlyimportant where the rFv is labeled for diagnosis or therapy employinghazardous labels, such as radionuclides, heavy metals, cytotoxic agents,and the like. Short residence times can also be important where the rFvis used to inhibit physiologically active materials in vivo e.g.hormones, enzymes, surface receptors, lymphocytes or other cells, andthe like.

The uniform composition allows for controlled labeling, enhancing theability to a conjugate label to a particular site on one or the other orboth of the chains. The uniformity permits controlled conjugations,accurate determinations of therapeutic activity, easy monitoring oftherapeutic effect, enhanced reproducibility of result and control andease of monitoring of side effects.

The subject method provides for accurate synthesis of polypeptide chainswhich can be brought together to form a binding site for a predeterminedepitopic site. The light and heavy chains prepared by the subject methodcan be brought together to bind to a particular ligand and may bebrought together in the presence or absence of the ligand. Also, themethod permits introducing a particular amino acid at either terminusfor particular applications e.g. tyrosine for radioiodination. By usingmonoclonal hybridomas as the source of the DNA for coding the variableregions, the naturally occurring binding efficiency is retained andbinding affinity can be widely varied.

Although the foregoing invention has been described in some detail byway of illustration and example for purposes of clarity ofunderstanding, it will be obvious that certain changes and modificationsmay be practiced within the scope of the appended claims.

What is claimed is:
 1. A specific binding composition ("rFv") comprisingtwo polypeptide chains having substantially the same amino acid sequenceof at least a portion of the variable region, without constant regionamino acids, of a mammalian immunoglobulin, said immunoglobulin havingbinding specificity to a predetermined ligand, wherein said polypeptidechains are prepared by expression of a DNA sequence coding for thevariable region, said expression occurring in the absence of expressionof a DNA sequence coding for natively associated constant region, andwherein said two polypeptide chains combine to form the rFv which has ahigh affinity and specificity for said predetermined ligand.
 2. Acomposition according to claim 1, wherein said two polypeptide chainsare the light chain of from about 95 to 115 amino acids and the heavychain of from about 110 to 125 amino acids, wherein said heavy chainincludes the D region.
 3. A composition according to any one of claims 1or 2, wherein each of said chains includes at least two cysteinesseparated by from about 60 to 70 amino acids and joined together througha disulfide link.
 4. A composition according to any one of claims 1 or 2wherein said rFv is labeled with a functionality capable of producing adetectable signal.
 5. A composition according to any one of claims 1 or2 wherein said rFv is labeled with a cytotoxic agent.
 6. A compositionaccording to claim 5, wherein said cytotoxic agent is a radionuclide.